The present invention relates to a color image pickup system for a color television system, and also relates to the color filter for that purpose. The present invention can be utilized in an image pickup system using a solid state image pickup element like a charge coupled device (CCD).
There have been three known systems for color image pickup, a three-tube system, a twin-tube system, and a single-tube system. In a three-tube system, the color of an image is divided to three elementary colors through a spectrofilter, and each color element of the image is illuminated on the screen of an image pickup tube. In a twin-tube system, the brightness is picked-up by the first tube, and the color hue (the combination of red and blue, or red and green) is picked-up by the second tube. While in a single-tube system, both the brightness and the color are picked-up by a single tube. Generally, the three-tube and twin-tube systems are better than the single-tube system as far as picture quality is concerned. However, said two systems have the disadvantage that the characteristics of the two or three pickup devices must be exactly equal, and the structure of the apparatus is complicated. It should be appreciated that it is rather difficult to obtain a plurality of image pickup devices of the same characteristics. On the other hand, a single-tube system has provided both the brightness signal and the color signal for a color television system by utilizing a color stripe filter shown in FIG. 1(A) or FIG. 1(B), or by controlling the pickup characteristics of the pickup cells of the pickup tube. In FIGS. 1(A) and 1(B), "C" shows cyan "Y.sub.1 " is yellow, "R" is red, "B" is blue, "G" is green, "B.sub.1 " is black, and "Y" is transparent. In FIG. 1(A), a filter has a transparent substrate (Y) with stripes of yellow (Y.sub.1) and inclined stripes of cyan (C). The scanning by an image device is performed horizontally along the arrow in the figure, and each scanning line picks up the color components cyan, white, yellow, etc. Those color components are differentiated by utilizing the frequency difference of those color components, or by applying a strobe pulse to the scanned signal to sample each color component from said scanned signal. In FIG. 1(B), four stripes of red(R), blue(B), green(G), and black(B.sub.1) are arranged as shown in the figure, and the scanning is performed along the horizontal line, therefore, the four components appear in the scanning line in sequence. Those color components are picked up by applying a strobe pulse to the scanned signal to sample the same and differentiate each color component.
However, the prior single-tube system shown in FIGS. 1(A) and 1(B) has the disadvantage that the picture quality or resolution, and the brightness of the picked up image are insufficient, since each picture cell is substantially composed of three or four color cells. That is to say, the resolution of the color television signal utilizing the color filter in FIG. 1(A) or FIG. 1(B) is only one third or one fourth of the resolution of a black-and-white television signal. Further, it should be noted that when a CCD device (charge coupled device) is utilized as an image pickup device, the resolution is not sufficient since the resolution or the number of the CCD device cells is not sufficient. Accordingly, the picture quality of a prior color television system utilizing a single image pickup device has not been acceptable.
Although we referred to three-tube systems, twin-tube systems or single-tube systems, those terms should be replaced by three-device systems, twin-device systems or single-device systems when a solid state image pickup device like CCD is utilized.